Electrostatic spraying



Oct 24, 1967 E. w. DRUM 3,348,965

ELECTROSTATIC SPRAYING Filed Feb. 4, 1964 10 2o 30 wv. AVAILABLE ONELECTRODE ADNI-IDHAB NOIlISOdBG NI mawaAoadwl 1N3aa3 INVENTOR EDWARD W.DRUM ATTORNEYS United States Patent 3,348,965 ELECTROSTATIC SPRAYINGEdward W. Drum, Indianapolis, Ind., assignor to Ransburg Electro-CoatingCorp., Indianapolis, Ind., a corporation of Indiana Filed Feb. 4, 1964,Ser. No. 342,488 7 Claims. (Cl. 117-93.4)

ABSTRACT OF THE DISCLOSURE Organic solvent based paint containingelectrically conductive metallic particles tends to short out when thepaint is mechanically atomized and sprayed through a zone in which theatmospheric ions are in a state of high spatial concentration to chargethe spray particles and improve paint deposition efficiency. Theconductive particles in the paint tend to become oriented to cause thisshorting to take place. When this condition is encountered, conductiveagents such as polar organic solvents or ionic surface active agents areincorporated in the liquid continuum of the paint to increase itsconductivity and cause the electrostatic potential to drain off throughthe paint to lower the potential which is available and thereby avoidthe shorting out incidental thereto.

The present invention relates to coating processes in which an organicsolvent-based paint containing dispersed electrically conductivemetallic particles is mechanically sprayed using an electronstaticallycharged surface or electrode in the immediate vicinity of the spray tocharge the sprayed paint and cause it to deposit with increasedefficiency, the paint being modified in accordance with the invention toavoid short-circuiting while preferably producing increased depositionefficiencies in comparison with the corresponding deposition efliciencyobtained by reducing the electrostatic potential below that level whichcauses short-circuiting.

The invention is particularly adapted to coating processes in which theliquid material is mechanically atomized and passed through a zone inwhich the atmospheric'ions are in a state of high spatial concentrationwhile the spray particles are also in a state of high spatialconcentration. This is conveniently accomplished by providing acompressed air spray gun having charging electrode means which create ahigh concentration of atmospheric ions in a zone close to the place offormation of the spray so that the material passing through the zone ofconcentrated atmospheric ions is also concentrated. Devices which sprayin this fashion are disclosed in the copen'ding applications of James W.Juvinall et al., Serial No. 60,657, filed October 5, 1960, now PatentNo. 3,169,882, and James W. Juvinall, Serial No. 148,793, filed October25, 1961, now Patent No. 3,169,883, the disclosures of which are herebyincorporated by reference.

It is of interest to note that the compressed air spray equipment ofcopending application Serial No. 60,657, now Patent No. 3,169,882,includes charging electrode means in direct electrical contact with partof the column of paint which is to be atomized so that the capacity ofthe column of paint to drain away a portion of the electrical potentialon the charging electrode will be immediately apparent. Indeed,structures in which the direct electrical contact noted above is presentare preferred in the invention because this permits adequate drain-offof electrical potential with minimum alteration of the paint. However,the invention is also applicable whenever mechanical force is used tomove material being sprayed or the particles in a state of high spatialconcentration through a zone in which the atmospheric ions are in astate of high spatial concentration even when the electrode chargingmeans is not in direct contact with any part of the column of paintwhich is supplied. This is because the close proximity between thecharging electrode and the column of paint supplied creates anelectrical continuity causing voltage drain-off through a conductivepaint.

As will be evident, the problem of the invention is the short-circuitingof the electrical potential through the column of paint which issupplied. Quite obviously, whenever the electrostatic spray structure issuch as to avoid the imposition of high electrical potentials on thecolumn of paint which is supplied, then the problem of the inventionwould not be present, and the directions provided by the invention wouldhave no purpose.

Electrostatic spray processes of the type under consideration have metwith substantial acceptance, but relatively few metallic paints could beused in the process, and these were usually paints containing lowconcentrations of metallic particles, such as powders or flakes. Whenthe metallic paints were formulated to contain high concentrations ofthe metallic particles, the electrostatic process would short out. Thiscan be attributed to an alignment or linkage of the metallic particlesoccurring as high voltage is passed from the electrode, down the paintcolumn to the grounded handle of the gun. Thus, once this linkageoccurred, a metal path was provided for all of the voltage to ground,hence, shorting out.

It has been noted in past experiments that a reduced voltage on theelectrode would allow continuous spraying without shorting out. Thevoltage applied then would have to be determined by the shorting voltageat which the particular metallic paint being used occurred. In general,the heavier the metallic concentration, the lower the applied voltagethat could be used.

In other words, some of the more desirable and more heavily pigmentedmetallic paints become too conductive under the influence of highvoltage so that the electrostatic forces, in the mechanical spray systemunder consideration, short out through the paint instead of improvingtransfer efficiency from the spray gun to the target. Accordingly, andwhen such paints are encountered, it is possible to proceed by greatlyreducing the voltage supplied to the electrode, but equipment for thispurpose is not usually available and, more important, significantvoltage reduction in this manner greatly lowers the depositionefliciency of the system.

Referring to standard voltages used in electrostatic spray systems ofthe type under consideration, voltages varying from about 60 kv. toabout kv. are common, 60 kv. being the voltage which is most usuallyemployed and the voltage supply equipment is normally designed to supplythis voltage without capacity for regulation. Many of the desirablemetallic paints will short out at this standard voltage and, indeed,will continue to short out until the voltage is lowered below 35 kv.,and even below 25 kv. Now, if one could avoid the shorting problem bydecreasing voltage a small amount, e.g., from 60 kv. to 40 kv. orhigher, then the decrease in transfer efficiency might be tolerable,since for this range of applied voltage, the decrease in transferefiiciency is a modest one. However, further losses of transferefliciency with further decrease in applied voltage becomes intolerablemaking it necessary to find other techniques to permit efficientelectrostatic spray operation.

It is also known that electrostatic spray efi'lciencies are affectedlbythe conductivity of the paint being sprayed since paints which areexcessively conductive do not respond well. In the electrostaticmechanical spray system under consideration and applying the standardkv. potential to paints free of metallic pigments, it is noted that whenthe paint conductivity is high enough to cause a portion of the chargeto drain off to ground through the.

paint to reduce the voltage on the electrode, that small reductions inelectrode voltage, e.g., from 60 kv. to 50 kv. are accompanied by arapid reduction in transfer efficiency. Using a conventional paint whichwill accept 60 kv., one can thin to a given viscosity using a highlyconductive solvent such as nitroethane, or a nonconductive solvent suchas xylene or mineral spirits. When the paint is thinned withnonconductive solvent and the voltage reduced to 50 kv. by operation ofa variable power control on the power supply, the reduction from 60 kv.to 50 kv. is accompanied by only a small reduction in transferefiiciency. Using conductive solvent such as nitroethane to reduce theelectrode voltage to 50 kv. causes a significantly greater reduction intransfer efficiency.

Accordingly, there is good reason to avoid the incorporation ofconductive solvents and additives to the liquid continuum of a paintsystem when such incorporation leads to an increase in the conductivityof the paint continuum which causes a significant drain-off to ground ofthe applied voltage. The term drain-off is used as a term of convenienceto denote a lossof voltage due to current flow through the paint.

Curiously, and preferred practice of the present invention is based uponthis finding, when metallic paints comprising metal particles suspendedin an organic solventbased paint continuum and which have a pronouncedtendency to short out at voltages which are well below standardelectrostatic spray voltages (4O kv. or lower) arelmodified to increasethe conductivity of the paint continuum so that about half or more ofthe applied voltage drains off to ground through the conductive paint,that the problem of shorting out may be avoided by the so-loweredelectrode voltage. Moreover, the transfer efficiency obtained bylowering electrode voltage in this manner is markedly superior to thetransfer efficiency obtained by adjusting the power supply to producethe same lowered electrode voltage and without increasing theconductivity of the paint continuum, even when equipment to control thepower supply is available.

There is presented herewith a graph which shows the improved depositionefliciency obtained by the invention.

The graph was obtained by taking numerous commercial metallic paintswhich tend to short out at 60 kv. and thinning these to desired sprayviscosity with xylene, and then spraying these using a variable powercontrol on the power supply to reduce the electrode voltage untilshorting out was avoided. The same commercial metallic paint was thenthinned to substantially the same spray viscosity using a mixture ofxylene and a conductive agent such as nitroethane to obtain a moreconductive paint producing a drain-off of voltage during use whichlowers the electrode voltage from about 60 kv. to about the same levelwhich avoided shorting out when the variable power control was reliedupon. In each instance, tests were run to determine how much paint wasdeposited on a target under given conditions and, using power controlvariations as a standard, the percent of improvement or lack ofimprovement was calculated and graphed against the electrode voltagewhich pertained in the comparison.

The curve which is presented represents an average of many runs withmany different paints.

In these tests, a specific grounded target is selected and exposed tothe spray for a given period of time, after which one measure the weightof. material which is deposited upon the target. It will be understoodthat the exact extent of improvement will vary with the specific testprocedure adopted, just as it will in actualpractice; but the fact ofimprovement is essentially independent of details of the test procedure.Accordingly, and while the specific percentage of improvement reportedmay have little meaning in an actual situation, any positive percentageis indicative of improved electrostatic deposition, and the greater thepercentage, the greater is the extent of improvement. In other words, ifa given test shows a 10% improvement by the use of the invention, thiswould not determine whether a 5, 10, or 15% improvement in depositionwould be obtained in any given industrial situation, though it woulddetermine that an improvement would be obtained.

The marked improvement in deposition efficiency up to about 35 kv.should be self-evident from the graph, especially in the range of from10-25 kv. which represents paints more poorly sprayable by themechanicalelectrostatic system under consideration.

From the standpoint of safety and reliability, the provision ofequipment on the power supply capable of regulating the level of theelectrostatic potential which is generated is not desirable. In suchinstance, the present invention is useful to avoid shorting out evenwhen the electrode potential at which this valuable result is obtainedis above 35 kv. or close to that potential which is normally generated.Thus, and despite the fact that there is a direct technique of somewhatsuperior efficiency possible, the fact that it is seldom available makesthe invention of value even when a small reduction in electrodepotential is all that is necessary.

Also, and from the standpoint of equipment, it should be observed thatusing more conductive paints in accordance with the invention causes asteady drain-off of voltage through the column of paint whenever voltageis applied. When the voltage is only applied while spraying takes place,the problem of overheating resistive components of the gun is notsignificant. Of course, if no at rangement is made to cause the appliedvoltage to be turned on andoif with the mechanical spray means, thencare must be taken to manually correlate the imposition of electricalpotential with mechanical spraying or resistive components capable ofwithstanding higher temperatures should be used or heat-dissipatingmeans employed. Any combination of these mechanical techniques to avoidoverheating may be used as desired.

It should be understood that increasing the conduc tivity of the paintcontinuum to cause a partial drain-off of electrode potential may causesome lowering of the electrode potential at which shorting out takesplaces, but this effect is small and does not significantly detract fromthe value of the invention. In some instances the opposite eifect may benoted and this is advantageous when it takes place.

As should be evident from the. foregoingflhe present invention is basedupon a phenomenon associated with the interaction between anelectrostatic field established by the potential on an electrode, a zoneof spray particles in a state of high spatial concentration and whichare to be charged by the electrostatic field and the capacity of acolumn of paint to drain off part of the potential on said electrode byvirtue of the conductivity of its liquid continuum. While the precisenature of this interaction may not be fully apparent, it is clear thatthe specific nature of the agent which is dissolved in the organicsolvent continuum of the paint to provide the desired increasedconductivity is not material so long as it is not injurious to theperformance of the paint.

Thus, any polar solvent may be used so long as it is compatible with theorganic solvent continuum of the paint. Similarly, any agent whichionizes to increase conductivity may be used so long as it is compatiblewith the organic solvent continuum of the paint and effective to causean increase in electrical conductivity. Surface active agents, eitheranionic or cationic, are effective for this purpose.

Other polar solvents are represented by 2-nitropropane, nitromethane,methyl ethyl ketone, acetone, acetonitrile, butyl acetate, butanol, etc.Cationic surface active agents which may be used are illustrated byquaternary and diquaternary ammonium halides having the formula:

in which R is preferably a hydrocarbon radical containing from -22carbon atoms and X is a divalent hydrocarbon chain of from 1-10 carbonatoms (preferably CH CH CH A partirularly preferred cationic surfaceactive agent is N-tallo-N,N-dimethyl, N,N',N-trimethyl-1,3-propylenediammonium chloride.

Appropriate anionic surface active agents are alkyl aryl sulfonates andphosphonates and the amine salts thereof such as the propyle amine saltof dodecyl benzene sulfonic acid.

It should also be understood that the paints which are the subject ofthe invention are those which become excessively conductive whensubjected to a unidirectional electrostatic potential of the level ofmagnitude contemplated herein and by virtue of highly conductivemetallic particles suspended therein which become oriented by theelectrostatic forces. Thus, the excessive conductivity of these paintsis not a characteristic of the paint continuum in the absence of themetallic particles.

Example The invention is illustrated by the specific example whichfollows in which a conventional long oil alkyd is pigmented withconventional aluminum paste to provide a metallic paint which presentsthe shorting out problem of the invention.

A raw paint is provided by intimately admixing: (1) 60.6 parts by weightof a long oil soya alkyd resin formed by reacting 1 mol of phthalicanhydride with 1.25 mols of glycerin and 1.75 mols of soya oil fattyacid; (2) 29.1 parts by weight of xylene; and (3) 10.3 parts by weightof a commercial aluminum paste constituted by a dispersion of nonleafingaluminum flake in hydrocarbon naphtha having a flash point of 105 C. anda Kau-ri-butanol value of 60, the paste containing 65% by weight of theflake aluminum. The nonleafing aluminum flake possesses a water coveringpower of 25,000 square centimeters per gram.

1000 cc. of this raw paint are thinned to 19 seconds (Zahn No. 2 Cup at80 F.) with 400 cc. of xylene. The above paint thinned with xylene asindicated can be electrostatically sprayed without danger of shortingout with an available electrode voltage of 27.3 kv. The same raw paint,when thinned to 18.5 seconds (Zahn No. 2 Cup at 80 F.) by combining 1000cc. of raw paint with 450 cc. of nitroethane and 30 cc. of thepropylamine salt of dodecyl benzene sulfonic acid, provides sufiicientconductivity to cause an automatic reduction in the electrode voltage to29.0 kv. from a 60.3 kv. power supply and was sprayable without dangeror shorting out at this electrode voltage.

An improvement in deposition efliciency of 16.8% is achieved by usingincreased paint conductivity to provide automatic voltage reduction incomparison with the utilization of a variable power supply to provide asimilar lowered electrode voltage to permit the spraying of the lessconductive paint thinned with xylene.

From the standpoint of completeness, the two experiments referred toabove were performed using a commercially available electrostatic spraygun available from the Ransburg Electro-Coating Corporation andidentified as Ransburg 301-E R-E-A Gun with a Ransburg 229F powersupply. The R-EA Gun referred to hereinbefore is constructed inaccordance with the teachings of said copending application Serial No.60,657, now Patent No. 3,169,882.

It should be observed that the paint rendered conductive is sprayed atan electrode voltage of 29.0 kv., whereas the paint of lesserconductivity is sprayed at 27.3 kv. Aside from the fact that this smallincrease in electrode voltage would not account for the approximately17% improvement in deposition efiiciency which is obtained, the reasonwhy the nonconductive paint is not sprayed at 29.0 kv. is because ittends to short out at this higher electrode voltage. As a result ofthis, it is necessary to run it at the highest voltage possible in theabsence of shorting out, e.g., at the 27.3 kv. value reportedhereinbefore.

As has been observed previously, in some instances, the paints modifiedfor increased conductivity will spray without shorting out at a higherelectrode voltage than the unmodified paints, and sometimes at a lowerelectrode voltage than the unmodified paint. In either event, thepresent invention provides improved operation. The improvement issomewhat greater where, as in the present example, modification of thepaint for increased conductivity permits application of a higherelectrode voltage without shorting out.

The invention is defined in the claims which follow.

I claim:

1. In a coating process in which a liquid organic solvent-based paint ismechanically atomized to form spray particles and in which the materialbeing sprayed is in a state of high spatial concentration when passedthrough a zone in which the atmospheric ions are in a state of highspatial concentration to cause the spray particles to becomeelectrostatically charged to thereby improve the efiiciency ofdeposition of the paint being sprayed upon a target, said paintcontaining electrically conductive metallic particles dispersed in anorganic solvent-based liquid continuum and said particles tending tobecome oriented to cause the electrostatic potential generating saidatmospheric ions to short out through the paint and the electrostaticpotential used being sufficient to short out through said paint, theimprovement which comprises incorporating in said liquid continuum ofsaid paint an agent which increases the conductivity thereof to causesaid continuum to become sufficiently conductive so that a portion ofsaid electrostatic potential drains off through the paint to therebylower the electrostatic potential to a level incapable of causing saidorientation of conductive particles and the shorting out incidentalthereto.

2. The improvement recited in claim 1 in which said agent is a highlypolar organic solvent.

3. The improvement recited in claim 1 in which said agent is an ionicsurface active agent.

4. The improvement recited in claim 1 in which at least about half ofthe electrostatic potential which is supplied is drained oflf throughsaid paint.

5. In a coating process in which a liquid organic solvent-based paint ismechanically atomized to form spray particles and in which the materialbeing sprayed is in a state of high spatial concentration when passedthrough a zone in which the atmospheric ions are in a state of highspatial concentration to cause the spray particles to becomeelectrostatically charged to thereby improve the efficiency ofdeposition of the paint being sprayed upona target, said paintvcontaining'electrically conductive metallic particles dispersed in vanorganictsolvent-based liquid continuum and saidtpart-icles tending tobecome oriented to cause electrostatic potentials up to about 40 -kV.generating said atmospheric ions to-short out through the paint, theimprovement which comprises incorporating in said liquidcontinuum-ofsaid paint an agent which increases the conductivity thereof to causesaid continuum tobecome sufi-iciently conductive :so that a portion ofsaid'electrostatic potential drains offthrough the paint to therebylower the electrostatic potential to a level of less than about 35 kv.,said level being incapable of causing said orientation of conductiveparticles and the shorting out incidental thereto.

6. The improvement recited in claim 5 in which particles tend to becomeoriented to cause electrostatic potentials up to about 30 kv. generatingsaid'atmospheric ions References Cited UNITED STATES iPATENTS 3,129,1124/1964 Marvin 117-934 3,130,066 4/1964 Brady 11793.43 3,210,316 10/1965'Merck et a1 ll793.4 X 3,251,551 5/1966 Walberg 117-934 X 15 ALFRED L.LEAVITT, Primaly Examiner.

A. GOLIAN, Assistant Examiner.

1. IN A COATING PROCESS IN WHICH A LIQUID ORGANIC SOLVENT-BASED PAINT ISMECHANICALLY ATOMIZED TO FORM SPRAY PARTICLES AND IN WHICH THE MATERIALBEING SPRAYED IS IN A STATE OF HIGH SPATIAL CONCENTRATION WHEN PASSEDTHROUGH A ZONE IN WHICH THE ATMOSPHERIC IONS ARE IN A STATE OF HIGHSPATIAL CONCENTRATION TO CAUSE THE SPRAY PARTICLES TO BECOMEELECTROSTATICALLY CHARGED TO THEREBY IMPROVE THE EFFICIENCY OFDEPOSITION OF THE PAINT BEING SPRAYED UPON A TARGET, SAID PAINTCONTAINING ELECTRICALLY CONDUCTIVE METALLIC PARTICLES DISPERSED IN ANORGANIC SOLVENT-BASED LIQUID CONTINUUM AND SAID PARTICLES TENDING TOBECOME ORIENTED TO CAUSE THE ELECTROSTATIC POTENTIAL GENERATING SAIDATMOSPHERIC IONS TO "SHORT OUT" THROUGH THE PAINT AND THE ELECTROSTATICPOTENTIAL USED BEING SUFFICIENT TO "CHORT OUT" THROUGH SAID PAINT, THEIMPROVEMENT WHICH COMPRISES INCORPORATING IN SAID LIQUID CONTINUUM OFSAID PAINT AN AGENT WHICH INCREASES THE CONDUCTIVITY THEROF TO CAUSESAID CONTINUUM TO BECOME SUFFICIENTLY CONDUCTIVE SO THAT A PORTION OFSAID ELECTROSTATIC POTENTIAL DRAINS OFF THROUGH THE PAINT TO THEREBYLOWER THE ELECTROSTATIC POTENTIAL TO A LEVEL INCAPABLE OF CAUSING SAIDORIENTATION OF CONDUCTIVE PARTICLES AND THE SHORTING OUT INCIDENTALTHERETO.